Mix 50 g active sourdough starter with 200 g lukewarm water
Add 350 g bread flour and 10 g salt
Mix until no dry flour remains
Rest dough 30 minutes
Perform 4 sets of stretch-and-folds every 30 minutes
Cover and bulk ferment at room temperature until doubled in size and bubbly (about 4–8 hours)
Pre-shape into a tight round or oval
Rest 20 minutes
Shape into a tight boule or batard
Place seam-side up in a floured banneton
Proof in the refrigerator 8–16 hours
Preheat oven to 250°C/500°F with a Dutch oven inside for at least 30 minutes
Transfer dough to parchment, seam-side down
Score the top with a sharp blade
Bake covered 20 minutes at 250°C/500°F
Reduce heat to 230°C/450°F and bake uncovered 20–30 minutes until deep golden
Cool on a rack at least 1 hour before slicing